Electrical discharge machining (EDM) is used for machining a hard material that can't be machined by a conventional method, and is usually used to machine conductive materials, so as to machine a cavity or profile on a workpiece, such as tool steel and metal alloy. The principle of EDM is applying a voltage pulse, which varies rapidly and periodically, between a conductive tool electrode and a workpiece. When sparking, the discharge area generates local high temperature, then melts and vaporizes surface metal of the workpiece, thereby removing the small part of metal.
The EDM may be generally classified into an ISO-frequency machining method and an ISO-energy machining method. In the ISO-frequency machining method, a pulse duration and an off time of a pulse voltage are fixed and are not changed along with discharge situations. In the ISO-energy machining method, the ignition state is that an open-circuit voltage is provided to the electrode and workpiece by a discharge loop, a gap voltage is detected at any time. The machining state is started after sparks are generated and the gap voltage drops, at this time, the timer is started. After a period of the machining time, the off state is start and the discharge loop is turned off to stop discharging for a period of off time. Afterward the ignition state repeats, the open-circuit voltage provided by the discharge loop to perform ignition. The total procedure is repeated in this way, and the discharge energy of each machining state is the same in this mode, thereby having a desirable machining efficiency. In the ignition state, an open-circuit voltage, for example, 90 V, is provided, and when generating sparks, the gap voltage drops dramatically to a low voltage, indicating that the ignition happened, and the low voltage is referred to as “arc voltage”. In conventional implementation, a reference voltage Vref is set, and when the gap voltage is less than the reference voltage during the ignition state, it indicates that the ignition happened, so that timing of the EDM is started. The conventional EDM is mainly used for machining metal conductor molds, an arc voltage of a metal conductor is about 20-30 V, so the reference voltage is generally fixed to about 50-60 V, or the reference voltage level Vref may be further adjusted by using a resistance that could be modulated, but the reference voltage level Vref is generally set to about 50-60 V.
Recently, there is a tendency of EDM development towards special material having resistance or composite materials, such as polycrystalline diamond (PCD), silicon carbide, and semiconductor. The conventional EDM cannot be performed on a polysilicon material and a silicon carbide material, because ignition discharge detection cannot be performed. Each special material, such as the silicon carbide or silicon wafer, has different material characteristics, and has a different arc voltage when the ignition happened. For example, the metal conductor has the arc voltage of 20-30 V, the PCD has the arc voltage of 20-60 V, the polysilicon material has the arc voltage of 120-160 V, and the silicon carbide has the arc voltage of 90-110 V at an initial period of sparking, and has the arc voltage of 20-50 V at a stabilized period. Moreover, in the ignition state, the open-circuit voltage should be higher than the arc voltage, so as to perform the discharge machining. Therefore, when cutting special materials, the arc voltage of the material should be known, so as to adjust the open-circuit voltage and reference voltage for ignition.
Each material has different characteristics, and even for the same material, such as the PCD, polysilicon and silicon carbide, material resistance varies due to factors such as particle size and distribution uniformity, and therefore, the manner using a single ignition voltage and a single reference voltage cannot satisfy requirements of all material machining, and may cause error of ignition discharge detection, resulting in the machining cannot be performed. Alternatively, the resistance is manually adjusted by an operator to an appropriate reference voltage level; however, this method is inconvenient, and has potential risks in work safety.